Airplane engines are typically mounted below an aircraft wing or near the tail section by an engine mount. Mounts are usually provided for both the forward portion of the engine and the aft portion of the engine, so as to distribute the engine load. Typical engine mounts include several components. One of the components is a generally planar upper fitting that has a mounting platform located along its upper edge that is used to attach the engine mount to a support structure of the aircraft, e.g., a wing strut or tail pylon. Multiple clevises are located on the lower edge of the upper fitting and also along a portion of an engine casing. Multiple links, pinned in the clevises of both the upper fitting and the engine casing, connect the engine to the support structure. Similar engine mounts of this type are used at both the forward and aft portions of the engine.
Engine mounts are designed to handle a variety of loads, during all phases of flight. The loads include vertical loads (the weight of the engine plus maneuver loads), axial loads (caused by the engine's thrust), side loads (caused by wind buffeting, for example), and torsion loads (caused by the rotary operation of the engine, or by the loss of a turbine blade). An engine mount must also accommodate thermal expansion and contraction of the engine relative to the mount. The effect of thermal expansion and contraction is most significant during cruise phase. During cruise, thermal expansion and contraction can cause an appreciable shift in the direction of loads acting on an engine mount.
Almost all airplane engine mounts are designed to be failsafe, i.e., to prevent the engine from separating from the airplane. Failsafe operation is provided by a secondary, or backup, load-carrying system. Two types of secondary systems are common. The first type utilizes components of the thrust reverser (such as the translating cowl) to carry engine loads. The second type utilizes catcher links placed within the engine mount itself. Catcher links are additional links in the engine mount that are typically unloaded during normal operation. Should a primary (i.e., non-catcher) link fail, the catcher links are capable of cooperating with the remaining unfailed links to carry engine loads. Link failures may result from many causes, including failure of pins or clevises; broken, deformed, missing, or mis-installed links; sheared pins; etc.
Between the two types of secondary systems, the thrust reverser system is the more widely used approach. On most airplanes, the use of catcher links is a more efficient solution, because they require relatively much less weight and space. Currently, relatively few catcher link engine mounts are known, and of these, not many describe three-link systems.
U.S. Pat. No. 5,275,357 (hereinafter referred to as "357") describes a three-link system, where the center link is the catcher link. The center link carries no load during normal operations, due to an oversized hole where the center link is attached to the engine casing. U.S. Pat. No. 5,303,880 (hereinafter referred to as "880") is similar to the device of the '357 patent, but with the addition of replaceable bushings. Although the devices disclosed in these two patents have three links, the systems are entirely different than the present invention. The most distinguishing difference is that the links in the '357 and '880 patents provide less horizontal and torsion load carrying capability than in the present invention. This is because the '357 and '880 devices do not have a dedicated horizontal and torsion load carrying member. The operation of the catcher link during a failed first or second link also produces a failure-mode load-couple (i.e., span of engine load contact points) that is smaller than the present invention. While the devices of the '357 and '880 patents appear sufficient, it is beneficial to have as large a span between the engine load contact points as possible during a failed link condition. These systems are also relatively tall, which makes them unusable on certain low-winged aircraft such as the Boeing 737.
U.S. Pat. No. 5,078,342 (hereinafter referred to as "342") also describes a three-link system, where the center link is the catcher link. The center link includes an arm that carries horizontal load during normal operations. A second arm of the catcher link is unloaded during normal operations due to an oversized hole therein. Although the device of the '342 patent has three links, it does not fully accommodate the horizontal and torsion loads during all failed link conditions. In particular, certain link failures are provided with only a torsion stop (i.e., two abutting metal faces), as opposed to an actual catcher link. Torsion stops tend to wear very quickly, and once worn, they are difficult or impossible to repair. Another disadvantage of torsion stops is that during failed link conditions, the torsion stops could shear themselves off due to the axial thermal growth of the engine. In addition, the device of the '342 patent is symmetrical, requiring that the strut and engine be hung normal to the ground in order to limit the engagement of the stops. However, the present invention is not sensitive to angle and can be hung other than normal to the ground (such as normal to the wing). The catcher link in the '342 device also has a failure-mode load-couple disadvantage, as described above regarding devices of the '357 and '880 patents.
Thus, there exists a need for a superior failsafe engine mount that provides link load carrying capability in all directions during normal and failed link operations. The ideal mount should be compact in height to provide greater ground clearance if used with an underwing engine, and compact in width in order to not significantly reduce bifurcation flow. The mount, however, should provide as wide a load-couple as possible during failed link conditions. The mount should not have any loose catcher links which vibrate and wear. The mount should provide adequate vertical and horizontal load bearing capability in the event of a single link failure, should be lightweight, and should allow installation of a vibration isolator if needed. As will be appreciated by the following description, the present invention is directed to providing such a superior failsafe engine mount.